REPORT OH THE EORAMINIFERA. 
641 
The genus Discorbina, as at present understood, was established by Parker and 
Jones for a group of Rotalince of which the Rotalia ( Trochulina ) turbo of d’Orbigny is 
the central type, and the Discorbites vesicularis and Rotalites trochidiformis of Lamarck, 
names of still earlier date, prominent examples. The morphological characters of the 
group are simple and easily understood, and they do not, on the whole, exhibit quite the 
same range of variation as is to be met with in the collateral genera Planorbidina and 
Pulvimdina. 
The test of Discorbina is either free or parasitic ; it is invariably spiral and typically 
Rotaliform. Generally speaking, the superior or spiral face is convex or conical, the 
inferior nearly flat, the whole of the segments being visible in the former aspect, the final 
convolution only in the latter, and the margin is more or less angular. There are, 
however, a number of forms, of which Discorbina rugosci is an example, the two faces of 
which are nearly equally convex and the peripheral edge round and lobulated ; others, 
such as Discorbina biconcava, that are complanate, both sides being flat or slightly 
concave and the margin square ; and others again, like Discorbina saidcii, which are 
nearly flat on the superior, and convex on the inferior side. The number of convolutions 
varies from between one and two to four ; the total number of segments from seven to 
twenty or rather more. 
The superior aspect of the test does not differ materially from that of the collateral 
genera of Rotalince , the distinctive features being more especially connected with the 
form and arrangement of the segments as seen on the inferior side. On the inferior face, 
as a rule, only the last convolution is visible ; and the aperture of the shell takes the 
form of a slit or fissure at the umbilical margin of the terminal chamber. The actual 
opening, however, is seldom visible, being hidden (in typical specimens) by a lobe or 
tongue projecting from the edge of the segment. These “umbilical lobes” or valves are 
extremely variable, both as to shape and dimensions. In their fullest development they 
are separated by marked constrictions from the body of the segments, and form supple- 
mentary chambers, which not only mask the umbilicus, but spread radially almost to the 
periphery of the test, covering-in the septal depressions to a greater or less extent, and 
alternating with the primary segments, as shown in PI. LXXXVII. figs. 2 b, 4 b, and 8 b. 
Such specimens constitute the genus Asterigerina of d’Orbigny. But in the majority of 
cases the supplementary structures are much less conspicuous. Sometimes they form 
solid masses of shell-substance, filling the umbilicus, and marked externally with 
exogenous tubercles ; on the other hand, the lobes of the successive chambers are often 
little more than arched projections overhanging the successive orifices ; and in certain 
species they are still more rudimentary, and insufficient collectively to fill or cover the 
umbilical vestibule. In one form or other these Asterigerine lobes may almost always 
be recognised, but not only do the several species differ in the degree to which they are 
developed, but individuals of the same species vary greatly in the same particular. 
